1. Field of the Invention
The present invention relates to a SPECT (Single-Photon Emission Computed Tomography) apparatus for detecting gamma rays emitted from an RI (radioisotope) given to an object by a detector such as a gamma camera to obtain detection data and reconstructing an image using the detection data to obtain a density distribution tomographic image of the RI in the object, and more particularly to a fan beam SPECT apparatus for obtaining the detection data by using a fan beam collimator.
2. Description of the Background Art
As shown in FIGS. 1 and 2, in a conventional SPECT apparatus of this kind, gamma rays are detected by a fan beam collimator or a parallel beam collimator. When the fan beam collimator is used, the gamma rays emitted from an RI (radioisotope) administered within an object can be effectively and advantageously detected. For example, three fan beam collimators are arranged, as shown in FIGS. 3 and 4, gamma ray incidence surfaces 100a of the fan beam collimators are rotated around an object P such as a man to detect the gamma rays emitted by the RI given to the object over a range of 360 degrees. In FIG. 4, 101 denotes a rotation center of the fan beam collimators.
In order to reconstruct an image from fan beam data detected by the fan beam collimators, the fan beam data is converted into parallel beam data which is the same data as that detected by using parallel beam collimators (fan-parallel data conversion), and the image is reconstructed using the parallel beam data in a parallel beam data reconstruction system. Alternatively, an image can be reconstructed using the fan beam data detected by the fan beam collimators in a fan beam data reconstruction system.
In the former case, as shown in FIG. 5, the gamma rays emitted the object P are detected by using three fan beam collimators 100 arranged in a triangular form to obtain fan beam data, and after collecting all of the fan beam data, if necessary, a preprocess such as a uniformity correction, a central position correction, a space filter process or the like is carried out. Then, the preprocessed fan beam data is picked up, and a fan-parallel data conversion and a postprocess are carried out to obtain the parallel beam data. The image is reconstructed using the parallel beam data to obtain a density distribution tomographic image of the RI in the object. In the latter case, as shown in FIG. 6, the fan beam data detected by the fan beam collimators 100 are picked up and after, if necessary, carrying out the preprocess, the image is reconstructed according to a direct reconstruction algorithm to obtain reconstructed data.
In the above described conventional apparatus, the fan-parallel data conversion means that data corresponding to parallel beam data are picked up from the whole fan beam data and lacking parallel beam data are formed by an interpolation method. For instance, as shown in FIG. 7 wherein 101 and 102 denote a rotation center of a fan beam collimator and a focal point, respectively, a relation between fan beam data X.sub.F (.theta.,.psi.) and parallel beam data X.sub.P (.theta.,t) is satisfied in the following formula. ##EQU1## That is, when there is unlimited fan beam data, parallel beam data can be obtained from the fan beam data by using this formula. In practice, however, the parallel beam data obtained from the conversion of the fan beam data does not always exactly correspond to the parallel beam data, and the parallel beam data is obtained by interpolation.
In the conventional apparatus using the fan-parallel data conversion algorithm, the fan-parallel data conversion cannot be carried out without all of the fan beam data. Further, since the number of readings when collecting of the collection data in the fan-parallel data conversion is large, it takes a long time to obtain the parallel beam data, and this is improper to the SPECT apparatus capable of collecting a plurality of slices of data at the same time. Further, since new data is stored in a memory to renew the old data in the preprocess (when the old data is required, they are copied), the data processing is complicated, and the data processing time is apt to be long. On the other hand, when the direct reconstruction algorithm is used, the calculation of the data processing is very complicated, and the conventional SPECT apparatus including the parallel beam data reconstruction system can not be used easily.
Further, in a conventional SPECT apparatus, a matrix size and a data collection angle pitch (hereinafter referred to as an angle) of a projection image (fan beam projection image) produced using the fan beam projection data are determined to the same values as those of parallel beam projection data obtained by the fan-parallel data conversion of the fan beam projection data. In the conventional apparatus, the maximum matrix size of the fan beam projection image is 128.times.128. However, in order to improve a resolving power for obtaining a superior image, a pixel size should be reduced. Hence, the matrix size is to be enlarged to one step larger size such as 256.times.256.
However, when the matrix size of the fan beam projection image is enlarged to 256.times.256 in order to improve the resolving power, the data amount to be processed in the fan-parallel data conversion is very much increased, and a memory having a large memory size is required. Further, a processing speed is fairly reduced to, for example, approximately 1/4.